Compositions of polycondensed branched polyester polymers and aromatic polycarbonates, and the closed cell polymer foams made therefrom

ABSTRACT

A foamable polyester composition comprising a combination of blowing agent and mixed polymer composition comprising: 
     a) a semi-crystalline polyester composition comprising a polycondensed branched polyester polymer, and 
     b) an aromatic polycarbonate polymer. 
     The foamable composition has a high extrusion melt strength, and can pulled across a die without ripping, to form a continuous thermoformable closed cell polyester sheet. The foamed sheets have a uniform, fine, and closed cell morphology; and simultaneously are less dense but have a higher impact strength than comparable closed cell polymers made in only with linear polyesters as the polyester component or without the aromatic polycarbonate.

This is a division of application Ser. No. 09/042,976 filed Mar. 17,1998, the entire disclosure of which is hereby incorporated byreference.

FIELD OF THE INVENTION

The invention relates to polyester polymer compositions and to theclosed cell polymer foams made therefrom. In particular, there isprovided a semi-semi-crystalline polycondensed branched polyesterpolymer blended with an aromatic polycarbonate to provide enhanced meltstrength to polyesters, and the closed cell polymer foams made therefromsimultaneously having improved impact strength and lower density.

BACKGROUND OF THE INVENTION

Foamed sheets of polyester are often made by extruding a polyestercomposition out of an annular die and over a "cooling can" after whichthe somewhat cooled polyester is cut to form a sheet that is taken up ona roll. A common problem encountered in extruding molten polyester ontoa moving belt is that the polyester extrudate tears as it is beingpulled over the cooling can. This type of failure is often due to thelack of adequate polyester polymer melt strength. A polyester polymerpossessing high melt strength resists the tendency of tearing or rippingwhen subjected to tensile forces. Accordingly, it would be desirable toincrease the melt strength of an extrudable polyester polymer to allow acontinuous take up of foamed sheet onto a roll and reduce the down timeneeded to re-align a torn extrudate onto or over a cooling can.

U.S. Pat. No. 4,544,677 is directed to foamable thermoplasticcompositions made with a mixture of polycarbonate resins, optionallypolyester resins, a foaming agent, and a nucleating agent having aspecific aspect ratio. The optional polyester resins disclosed arelinear, and additional amounts of acid or glycols could be added inamounts ranging from 0.5 wt. % to 50 wt. % based on the totalcomposition. We have found, however, that linear polyesters are not wellsuited to extruding foamed sheets because the resin composition hasinsufficient melt strength. Without sufficient melt strength and dieswell, the molten resin both rips and tears as it is pulled over or ontothe cooling can, and fails to yield a product with low density. Certainfoaming methods, such as injection molding techniques as described inU.S. Pat. No. 4,544,677, do not require the use of a resin of great meltstrength because the resin is not placed under tension or taken upduring the foaming of the resin. Accordingly, there remains a need todevelop a resin which has high melt strength.

U.S. Pat. No. 5,360,829 discloses foamed polyester sheets made fromamorphous polyesters, 20 to 100 parts by weight of a polycarbonate resinbased on 100 parts by weight of the polyester, and a nucleating agent inorder to impart to the foamed sheet a uniform cellular morphology and ahigh foaming ratio while maintaining its mechanical properties anddimensional stability. The patentees noted that when crystallinepolyesters were blended with polyolefins, the cellular morphology of theresulting foamed sheets was not uniform, and the foaming ratio was toolow. Accordingly, the patentees recommended using amorphous polyestersin combination with polycarbonates to overcome this deficiency.Amorphous polyesters, however, even in foamed compositions, suffer thedrawback in that they are excluded from being used in high temperatureapplications. It would be desirable to find an alternative polyestercomposition that possesses high melt strength without this noteddeficiency.

The foamed sheets of polyester find a number of consumer uses, many ofwhich subject the polyester foam sheet to high impacts. Therefore, it isalso desirable to make a polyester foam sheet which has high impactstrength. One way of doing so is to increase the density of thepolyester foam, either by increasing the amount of polymer per unitvolume, or by adding reinforcing agents. Neither method is an attractivesolution. Both increase the cost of the foam. Further, in the lattermethod, the amount of reinforcing agent that can be added is limited bythe increase in viscosity to the polymer melt resulting from thereinforcing agent.

Attempts to decrease the density of the polyester foam have also lead toa corresponding decrease in the impact strength of polyester foams. Itwould be highly desirable to make a polyester polymer that overcomesthis longstanding direct relationship between the foam density andimpact strength; that is, there exists a need to make a polyester foamthat simultaneously has a low density and an improved impact strength.Accordingly, in addition to making a polyester polymer which processeswell by virtue of its high melt strength, as an additional embodiment,it is desirable to make a polyester foam sheet, and a thermoformedarticle from the sheet, which has a good cellular structure, lowdensity, and high impact strength.

U.S. Pat. No. 5,502,119 suggests stabilizing blends of polyesters andaromatic polycarbonates with a particular organosilicate. The types ofpolyesters described therein are not polycondensed branched types ofpolyesters. U.S. Pat. No. 5,504,130 describes filled thermoplasticcompositions where about 15-50% of at least one poly(phenylene ether)resin is compatibilized with about 20-80% of at least one polyesterresin by using from 3-50%, preferably of from about 8-20%, of anaromatic polycarbonate having a weight average molecular weight of atleast about 40,000. These patents, however, do not address the problemof how to improve the melt strength of polyester compositions, or how toadditionally make foamed sheets having good cellular uniformity and highimpact strength at low density.

SUMMARY OF THE INVENTION

There is now provided a closed cell polymer comprising a mixed polymercomposition comprising:

a) a semi-crystalline polyester composition comprising a polycondensedbranched polyester polymer, and

b) an aromatic polycarbonate polymer.

This polymer mixture has a high melt strength and can be extruded andpulled across, over, or through a die with a low frequency of ripping ortearing.

In another embodiment of the invention, there is provided a mixedpolymer composition comprising:

a) a polyester composition in an amount of at least 90 wt. %, based onthe weight of all polymers in the mixed polymer composition, saidpolyester composition consisting of polyester polymers, said polyesterpolymers comprising polycondensed branched polyester polymers, and

b) aromatic polycarbonate polymers in an amount ranging from 0.01 wt. %to 10.0 wt. %, preferably from 0.01 wt. % to 2.0 wt. %, based on theweight of the mixed polymer composition.

There is also provided a foamable composition comprising a combinationof blowing agent and mixed polymer composition comprising:

a) a semi-crystalline polyester composition comprising a polycondensedbranched polyester polymer, and

b) an aromatic polycarbonate polymer.

Processes for extruding the mixed polymer compositions, making a foamedsheet and thermoforming the foamed sheet made with the mixed polymercomposition in the presence of a blowing agent are also included.

An unexpected further advantage observed in the closed cell polymers isthat they had a more uniform, fine, and closed cell morphology; and werethicker and less dense than the same closed cell polymers made only withlinear polyesters as the polyester component or without the aromaticpolycarbonate. The thermoformed sheets made from the foamed sheets usingthis composition also had a higher impact strength compared to the samethermoformed sheets made only with linear polyesters as the polyestercomponent or without the aromatic polycarbonate.

DETAILED DESCRIPTION OF THE INVENTION

The foamable composition comprises the mixed polymer composition,blowing agent, and any other optional additives, such as impactmodifiers, fibers, stabilizers, and flameretardants. The foamablecomposition for purposes of calculating amounts of each ingredientincludes the sum total of all ingredients used in the embodiment.

The mixed polymer composition comprises:

a) a polyester composition comprising a polycondensed branched polyesterpolymer, and

b) an aromatic polycarbonate polymer; along with any optionalhydrocarbon polymers and oligomers having only carbon, hydrogen, oxygen,and nitrogen, other than impact modifiers.

The polyester composition comprises a polycondensed branched polyesterpolymer and other polyester polymers.

The closed cell polymer, and the mixed polymer compositions of theinvention, are thermoplastic. The mixed polymer compositions, due totheir thermoplastic character, can be pelletized and subsequently meltextruded, optionally, further advanced.

A. The Polyester Composition

The a) polyester composition may contain aromatic polyesters, aliphaticpolyesters, cycloaliphatic polyesters, or mixtures thereof. Thepolyester polymers in the polyester composition are preferably derivedfrom at least 50 mole % or more of aromatic polycarboxylic acids, basedon the total number of polycarboxylic acids used, more preferably 80% ormore, most preferably 100%, based on the weight of all polycarboxylicacids.

The a) polyester composition in one embodiment is semi-crystalline, andit preferably is semi-crystalline when one employs only 10 wt. % or lessof an aromatic polycarbonates and at least 90 wt. % of polyesterpolymers. By a semi-crystalline polyester composition or asemi-crystalline polyester polymer is meant one which has a degree ofcrystallinity of at least 15%, cooled to and measured at 25° and atabout 1 atmosphere. The % crystallinity is conveniently determined byDSC, Differential Scanning Calorimetry. In another preferred embodiment,the degree of crystallinity is 20%-40%, more preferably 25% to 35%, eachat 25° C. In other more preferred embodiments, the degree ofcrystallinity is at least 15% at 40° C., most preferably at least 15% at60° C. It is within the scope of one embodiment of the invention,however, to provide a mixed polymer composition comprising a polyestercomposition, which may be amorphous for applications which are notsubjected to high temperatures.

The mixed polymer composition comprises an a) polyester composition atleast a portion of which must contain polycondensed branched polyesterpolymers. The polycondensed branched polyester polymers may be aromatic,aliphatic, or cycloaliphatic. By a polycondensed branched polyesterpolymer or a polycondensed branched polyester composition is meant apolymer or a composition containing polymers derived frompolycondensation branching agents having active hydrogen functionalitieseffective to form condensation linkages. While the type of functionalityof the polycondensation branching agent may include amines, isocyanates,or thiols, the preferred type of functionality on the polycondensationbranching agent is a hydroxyl group or a carboxylic acid group to formester-linkages. The polycondensation branching agent has an averagefunctionality of greater than two, preferably greater than 2.5, morepreferably 3.0 or more. The polycondensation branching agent provides ameans to create a molecule which contains a number of high molecularweight branches, e.g. on the order of 5,000 M_(w) or more, the number ofbranches corresponding to the number of functional sites on thepolycondensation branching agent. The presence of these polycondensedbranched polymers improves the melt strength of a molten polyestercomposition exiting a die head, and further improves the impact strengthof the resulting foam.

Accordingly, one may react a polycarboxylic acid containing an averagecarboxylic acid functionality of greater than two, with a diol monomer.Alternatively, dicarboxylic acids can be used to react with a polyolcontaining an average hydroxyl functionality of more than 2. Or, ifdesired, a mixture of di- and higher polycarboxylic acids can be made toreact with a mixture of di- and higher polyols.

The particular structure of the polycondensation branching agent is notlimited, so long as the average active hydrogen functionality is greaterthan 2. In general, those molecules having C₁ -C₁₉ alkyl, alkaryl,aralkyl, cycloaliphatic, or aromatic moieties are suitable. Preferredpolycondensation branching agents are the carboxylic acid and hydroxylfunctional compounds.

Illustrative examples of the polycarboxylic acids having an averagefunctionality of greater than 2 are benzenetricarboxylic acid, ,benzophenone tetracarboxylic acid, oxoisophthalic acid, pyromelliticacid, trimesic acid, trimellitic acid, citric acid, the anydrides orsalts thereof, or mixtures thereof. Dicarboxylic acids may also be mixedwith these acids to achieve the desired functionality. Instead of usingthe free acids, the corresponding polycarboxylic acid derivatives mayalso be used such as polycarboxylic acid mono-, di-, tri-, or higheresters of alcohols with 1 to 4 carbons.

Illustrative examples of polyol monomers having a nominal functionalityof greater than two are trimethylolethane, 1,2,6-hexanetriol, alpha-methyl glucoside, glycerine, sucrose, glucose, mannose, fructose,trimethylol propane, mannitol, sorbitol, pentaerythritol, and the highermolecular weight polyoxyalkylene polyether adducts made by reactingthese polyols with alkylene oxides. The higher functional polyols can bemixed together to provide blends having the desired functionality, suchas blends of glycerine and sucrose, or blends of pentaerythritol andglycerine or trimethylol propane. Alternatively, the higher functionalpolyols can be mixed with diols to provide a blend having the desiredfunctionality, such as ethylene glycol blends with glycerine,pentaerythritol, and/or sucrose. Suitable diols for admixture are thosementioned below with respect to making linear polyester polymers.

The amount of the polycondensation branching agent used is effective toimprove the melt strength of the composition at the die head. Suitableamounts of the polycondensation branching agents in the polyestercomposition range from 0.01 to 5.0 mole% of polycondensation branchingagent based on the total moles of all monomers used to make the a)polyester composition. The large improvement in melt strength isobserved using amounts of polycondensation branching agent as low asfrom 0.05 to 1.0 mole%. On a weight percentage, the amount of thepolycondensation branching agent is preferably less than 1.0 wt. % basedon the weight of the a) polyester composition, more preferably 0.2 wt. %or less, most preferably 0.15 wt. % or less, especially when thepolycondensation branching agent has an average molecular weight of 150or less. Without being bound to a theory, it is believed that the smallmolar amount of polycondensation branching agent relative to the molesof other polyester forming monomers, coupled with the lack of freereactive sites on the growing independent polymer chains, favorspolyester chain growth through acid-glycol reactions across thepolycondensation branching agent reactive sites and continuing outwardacross the growing polyester branch, over crosslinking reactionsoccurring between the reactive sites on polycondensation branchingagents and inner portions along the independently developing polyesterpolymer molecules. Accordingly, the polyester composition maintains itsthermoplastic character.

The remainder of the a) polyester composition comprises any of the knownpolyester polymers. Other suitable polyester polymers are those whichcontain repeat units which are derived from diacids other thanterephthalic acid and/or glycols in addition to or other than ethyleneglycol. For instance, other suitable acids include isophthalic acid,naphthalenic dicarboxylic acid, 1,4-cyclohexane dicarboxylic acid,1,3-cyclohexane dicarboxylic acid, succinic acid, glutaric acid, adipicacid, sebacic acid, 1,12-dodecane dioic acid, and the functional acidderivatives thereof such as the dimethyl, diethyl, or dipropyl esters ofthe dicarboxylic acids. Further, the anhydrides or the acid halides ofthese acids may be employed

In addition to the commonly used ethylene glycol and diethylene glycolmonomer for making an aromatic polyester polymer, other suitable diolresidues include, in addition to or replacing, but preferably only up to20 mole percent, the linear diols such as propylene glycol, 1,3 propanediol, triethylene glycol, 2,4-dimethyl-2-ethylhexane-1,3-diol,2-ethyl-2-butyl-1,3-propane diol, 2-ethyl-2-isobutyl-1,3-propane diol,1,3-butane diol, 1,4-butane diol, 1,5-pentane diol, 1,6-hexane diol,1,2-, 1,3- and 1,4-cyclohexane dimethanol,2,2,4,4-tetramethyl-1,3-cyclobutane diol, 1,4-xylylene-diol, and thelonger chain diols and polyols.

Any method of making the a) polyester composition is included within thescope of the invention. Conventional methods include reacting the acidor its ester derivative with the glycol/polyol component or its esterforming derivative under heat in the presence or absence of anester-interchange catalyst or an esterification catalyst, and thenheating in the presence of a polymerization catalyst. The polycondensedbranched polyester polymer may be separately manufactured, isolated, andsubsequently added to the polyester composition, or the a) polyestercomposition can be branched in situ by adding the polycondensationbranching agent to the monomers during the manufacture of the polyestercomposition, preferably the latter for ease of handling and processing.

The operating conditions for the polyester polymerization are generallyknown to those skilled in the art of preparing polyester polymers andcopolymers. The polymerization temperature is generally from about 250°C. to about 295° C., depending upon the monomer units present, and ispreferably operated in the range of 265° C. to 285° C. The pressure usedfor the polycondensation reaction is gradually reduced-over the courseof the reaction, from atmospheric pressure to a high vacuum of less than1 torr. In another embodiment, the polyester polymer may be made undertransesterification conditions, which are also known to those in theart.

Suitable polymerization catalysts include the compounds of lithium,sodium, potassium calcium, barium, magnesium, manganese, cobalt,germanium, antimony, lead, tin, copper, titanium, palladium, platinum,gold, or silver. Preferable as catalysts are those compounds which canbe reduced to elemental form which may assist in the reduction of cycletimes in the mold or the time required to re-heat the polyester polymersin such operations as blow molding. Particularly preferable are antimonycompounds, such as antimony trioxide and antimony acetate.

Stabilizers can be added which include phosphoric acids, phosphorousacids, orthophosphoric acids, phosphonic acids, and derivatives of thesesuch as triphenyl phosphite, trimethyl phosphate, triphenyl phosphate,dimethyl mono-,-hydroxyethyl phosphate, mono-methyl di-βphosphate,tri-β-hydroxyethyl phosphate, phenylphosphonic acid, and dimethyldibenzylphosphonate. The preferred stabilizers are those which arecapable of reducing the polyester polymerization catalyst compound toits metallic elemental form.

The process for polymerization is preferably carried out with the use ofa "heel." The heel is an esterification product recycled to the initialstages of the esterification reaction to increase the solubility of thedicarboxylic acid, thereby increasing the reaction rate of thedicarboxylic acid and the diol. The use of a heel is explained in U.S.Pat. No. 4,020,049 (Rinehart), and may be applied to both continuous andbatch manufacturing processes.

The molecular weight of the copolymers produced by the process of thisinvention may be increased by polymerization in the solid state. Thesolid state polycondensation reaction is conducted at temperatures fromabout 190° C. to about 250° C., in the presence of an inert gas (e.g.nitrogen). The inert gas serves to remove reaction byproducts, such asexcess diol and water. The solid state polymerization reaction isgenerally continued until the polymer reaches an intrinsic viscosity of0.65 dl/g or more. The intrinsic viscosity of the polyester polymercomposition, as well as the intrinsic viscosity of the polycondensedbranched polyester polymer, suitably ranges from 0.65 to 1.75 dl/g, morepreferably from 0.95 to 1.5 dl/g.

The polyester copolymer produced in the melt polymerization processpreferably contains a carboxyl content that provides an enhanced solidstate polymerization rate. A method for producing polyester polymerswith an optimum carboxyl content is described in U.S. Pat. No. 4,238,593(Duh).

B. The Polycarbonates

Any conventional aromatic polycarbonate composition is suitable for usein the invention. Both linear and branched aromatic polycarbonates aresuitable for use in the invention. Linear aromatic polycarbonatesimprove the impact strength of foamed sheets. The impact strength offoamed sheets at both room temperature and at -20° C., however, can beimproved by using branched aromatic polycarbonates. The particularpolycarbonate chosen may depend upon the application and desiredproperties, but where the end use is in contact with food, FDA approvedpolycarbonates are the material of choice.

Aromatic polycarbonates are materials known per se. They are generallyprepared by reacting a dihydric phenol compound with a carbonateprecursor, for example, phosgene, a halogen formate or a carbonateester, according to the procedures and materials identified in U.S. Pat.Nos. 4,098,750, 4,123,436, 3,169,121, and U.S. Pat. No. 3,153,008, eachincorporated herein by reference.

Aromatic polycarbonates are polymers that comprise units of the formula:##STR1## wherein A is a multivalent aromatic radical derived from thepolyhydric phenol used in the preparation of the polymer, and nrepresents the number of repeat carbonate units. Mononuclear orpolynuclear aromatic compounds which comprise two or more hydroxyradicals, which are each directly bonded to a carbon atom of an aromaticnucleus, may be used as polyhydric phenols in the preparation of thearomatic polycarbonates.

Examples of suitable dihydric phenols are:2,2-bis-(4-hydroxyphenyl)propane;2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane;2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane;2,2-bis(3,5-dichloro-4-hydroxyphenyl)propane; hydroquinone; resorcinol;2,2-bis-(4-hydroxyphenyl)pentane; 2,4'-(dihydroxy diphenyl)methane;bis(2-hydroxyphenyl)methane; bis-(4-hydroxyphenyl)methane;bis-(4-hydroxy-5-nitrophenyl)methane; 1,1-bis-(4-hydroxyphenyl)ethane;3,3-bis(4-hydroxyphenyl)pentane; 2,6-dihydroxy naphthalene;bis-(4-hydroxydiphenyl)sulfone;bis-(3,5-diethyl-4-hydroxyphenyl)sulfone;2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)propane; 2,4'-dihydroxyphenylsulfone, 5'-chloro-2,4'-dihydroxydiphenylsulforie; bis-(4-hydroxyphenyl)diphenyl sulfone; 4,4'-dihydroxydiphenyl ether;4,4'-dihydroxy-3,3-dichlorodiphenyl ether; 4,4'-dihydroxy-2,5-dihydroxydiphenyl ether. The aromatic polycarbonates can also be used as mixturesof the aforementioned polycarbonates. Particularly useful aromaticpolycarbonates are those derived from Bisphenol A.

The branched aromatic polycarbonates suitable for use in the inventionare any of those commercially available. Their method of manufacture areknown. In general, the branched aromatic polycarbonates may be made byreacting a carbonate precursor, for example, phosgene, a halogen formateor a carbonate ester with a branching agent having more than two activehydroxyl functionalities. Examples of branching agents having more thantwo active hydroxyl functionalities are the polyol branching agentsmentioned above with respect to the polycondensed branched polyesterpolymer. Alternatively, the branching on the branched aromaticpolycarbonate may be provided by employing a branching agent having onlytwo active hydrogen functionalities but with a highly branchednon-reactive alkyl, cycloalkyl, alkaryl, aralkyl, or aryl network suchas those described in U.S. Pat. No. 4,469,861, incorporated herein byreference. Examples of these branches are C₂ -C₁₀ alkyl, C₅ -C₁₅cycloalkyl, or C₆ -C₂₀ aryl, aralkyl, or alkaryl aromatic branches.

The term polycarbonate is also intended to embracepolyesterpolycarbonates (PCC). The PCC's for use in the invention areknown and some can be obtained commercially. Generally, they arecopolyesters comprising recurring carbonate groups and aromaticcarbocyclic groups in the linear polymer chain, in which at least someof the carboxylate groups and at least some of the carbonate groups arebonded directly to ring carbon atoms of the aromatic carbocyclic groups.These poly(ester-carbonate) copolymers, in general, are prepared byreacting a difunctional carboxylic acid, such as phthalic acid,isophthalic acid, terephthalic acid, homophthalic acid, o-, m-, andp-phenylenediacetic acid, 2,6-naphthalene dicarboxylic acid, mixtures ofany of the foregoing, and the like with a dihydric phenol and acarbonate precursor, of the types described above. A particularly usefulpolyester carbonate is derived from bisphenol-A, isophthalic acid,terephthalic acid, or a mixture of isophthalic acid and terephthalicacid, or the reactive derivatives of these acids such as terephthaloyldi-chloride, isophthaloyl dichloride, or a mixture thereof, andphosgene.

Aromatic dihydric phenol sulfone resins can also be used as the aromaticpolycarbonate. For example homopolymers of dihydric phenol, and adihydroxydiphenyl sulfone and a carbonate precursor can be prepared.

The Mixed Polyester Composition

The amount of aromatic polycarbonate in the mixed polymer composition iseffective to improve at least the melt flow strength of the mixedpolymer composition. Suitable amounts of the aromatic polycarbonaterange from 0.05 wt. % to 10 wt. % based on the weight of the mixedpolymer composition. However, we have found that amounts of aromaticpolycarbonate ranging from 0.1 to 2.0, more preferably 1.5 wt. % orless, based on the weight of the mixed polymer composition, areeffective to obtain a mixed polymer composition having excellent meltflow strength and impact strength when thermoformed into articles. In apreferred embodiment, one may add amounts of polycarbonate as low asfrom 0.2 to 1.0 wt. %, based on the weight of the mixed polymercomposition, to attain the benefits associated with improved melt flowstrength and impact strength.

In another embodiment of the invention, the mixed polymer compositioncomprises at least a polyester composition which consists of polyesterpolymers in an amount of at least 90 wt. %, more preferably 97 wt. % ormore, based on the weight of all polymers in the mixed polymercomposition, the polyester polymers comprising polycondensed branchedpolyester polymers; and an aromatic polycarbonate polymer in an amountof from 0.01 to 10 wt. %, more preferably 0.01 to 1.5 wt. %, based onthe weight of the mixed polymer composition. In the more preferredembodiment, the polyester polymers are the reaction product of from 0.01to 1.0 mole percent of the polycondensation branching agent.

The polyester composition and the polycarbonate are blended together. Inaddition to improved melt flow strength and impact strength, anotheradvantage of the invention lies in that the mixed polymer compositionremains essentially as a blend of the polycondensed branched polyesterand the aromatic polycarbonate even at extrusion processingtemperatures, which exceed the melt temperature of the resins, based onLC mass-spectrometer analytical techniques. Analytical screening of thearticles produced after foam extrusion using an LC mass-spectrometerfailed to detect the presence of transesterification products betweenthe polyester and the polycarbonate, indicating that the polyester andthe polycarbonate remain substantially as a blend throughout theirprocessing life without the production of detectable by-products.

Any method of blending is appropriate. One convenient method comprisesprecompouding by blending the polyester and polycarbonate resins andother ingredients in powder or granular form, extruding the blend andcomminuting into pellets or other suitable shapes. The ingredients arecombined in any usual manner, e.g., by dry mixing or by mixing in themelted state in an extruder, on a heat mill or in other mixers. Analternative method of blending can comprise preparing a preblend of thepolyesters and then adding the polycarbonate and other ingredients andadditives to the preblend at the extruder used to make a foamed sheet.Alternatively, each ingredient can be added separately by hopper at thefoam extruder.

The other ingredients that may be included within the meaning of a mixedpolymer composition include other hydrocarbon polymers and oligomershaving only carbon, hydrogen, oxygen, and nitrogen, such as nucleatingagents, but not impact modifiers. While some nucleating agent packagesmay include small amounts of polymers containing halogen atoms, thesepolymers are included within the meaning of a nucleant and within thescope of a mixed polymer composition.

D. The Foamable Composition

To the mixed polymer composition may be added those other additiveswhich are not polymers or oligomers exclusively made of carbon, oxygen,hydrogen, and nitrogen, such as blowing agents, fillers, flameretardants, uv- and other stabilizers, impact modifiers, anti-oxidants,drip retardants, dyes, pigments, colorants, antistatic agents,plasticizers and lubricants. These additives are known in the art, asare their effective levels and methods of incorporation. Their amountsvary depending upon the type of additive, but generally, the mixedpolymer composition will comprise at least 40 wt. % of all theingredients used to make an foaming composition, preferably at least 80wt. %, more preferably at least 95 wt. %.

The type of blowing agent used to foam the polymer is not limited.Common physical blowing agents include nitrogen gas, carbon dioxide gas,halogenated gases, propane, butane, pentane, hexane, helium, neon, argonand krypton. Inert gases can be used in extrusion foaming, and arepreferred because of their handling ease, environmental friendliness,and reduced cost.

Other physically active blowing agents are those which boil at extrusiontemperatures or less. These include volatile non-halogenatedhydrocarbons having two to seven carbon atoms such as alkanes, alkenes,cycloalkanes having up to 6 carbon atoms, dialkyl ethers, cycloalkyleneethers and ketones; hydrochlorofluorocarbons (HCFCs); hydrofluorocarbons(HFCs); perfluorinated hydrocarbons (HFCs); fluorinated ethers (HFCs);and decomposition products. Specific examples include butane, isobutane,2,3 dimethylbutane, n- and isopentane and technical-grade pentanemixtures, cyclopentane, n- and isohexanes, cyclohexane, n- andisoheptanes, n- and isooctanes, n- and isononanes, n- and isodecanes, n-and isoundecanes, and n- and isododecanes, and in particularcyclopentane and/or pentane. Perfluorocarbons or fluorinated ethersinclude hexafluorocyclopropane (C-216); octafluorocyclobutane (C-318);perfluorotetrahydrofuran; perfluoroalkyl tetrahydrofurans;perfluorofuran; perfluoro-propane, -butane, -cyclobutane, -pentane,-cyclopentane, and -hexane, -cyclohexane, -heptane, and -octane;perfluorodiethyl ether; perfluorodipropyl ether; and perfluoroethylpropyl ether. Suitable hydrofluorocarbons include difluoromethane(HFC-32); 1,1,1,2-tetrafluoroethane (HFC-134a);1,1,2,2-tetrafluoroethane (HFC-134); 1,1-difluoroethane (HFC-152a);1,2-difluoroethane (HFC-152), trifluoromethane; pentafluoropropane(245b); heptafluoropropane (R-227a); hexafluoropropane (R-136);1,1,1-trifluoroethane; 1,1,2-trifluoroethane; fluoroethane (R-161);1,1,1,2,2-pentafluoropropane; pentafluoropropylene (R-2125a);1,1,1,3-tetrafluoropropane; tetrafluoropropylene (R-2134a);difluoropropylene (R-2152b); 1,1,2,3,3-pentafluoropropane; and

1,1,1,3,3-peintafluoro-n-butane. One may also use chemical blowingagents which decompose on heating to release a gas such as N₂ or H₂.Examples include hydrazine derivatives such as azocarbanoamide,5-phenyltetrazole, sulfonly hydrazide, sulfonyl semicarbazide and sodiumborohydride.

In addition to blowing agents, foaming nucleating agents can be used,such as glass, talc, silica and mica. These agents have a lower specificgravity than metals, are inexpensive because they are mass produced, andstable quantities of uniform particle size are easily obtainable. Thesenucleating agents may be used either singly or in combination. Otherfoaming aids include organic acids, Ca, Zn, Mg, Ba, Al, Pb and Mn saltsof organic acids, and organic acid esters.

The amount of blowing agent is determined primarily by the desired sheetdensity and the efficiency of the blowing agent used. In general, theamount of blowing agent ranges from 1 pbw (part by weight) to 10 pbwbased on 100 pbw of the mixed polymer composition. The amount of blowingagent when using the mixed polymer composition can be reduced to make afoam sheet of the same density as compared to a foam sheet made withouta polycondensed branched polyester or without an aromatic polycarbonate.

To the mixed polymer composition may also be added flame retardants asneeded. Examples of suitable phosphate flameproofing agents aretricresyl phosphate, tris(2-chloroethyl) phosphate, tris(2-chloropropyl)phosphate, and tris(2,3-dibromopropyl) phosphate. In addition to thesehalogen-substituted phosphates, it is also possible to use inorganic ororganic flameproofing agents, such as red phosphorus, aluminum oxidehydrate, antimony trioxide, arsenic oxide, ammonium polyphosphate(Exolit Registered TM ) and calcium sulfate, molybdenum trioxide,ammonium molybdate, ammonium phosphate,pentabromodiphenyloxide,2,3-dibromopropanol, hexabromocyclododecane,dibromoethyldibromocyclohexane, expandable graphite or cyanuric acidderivatives, e.g., melamine, or mixtures of two or more flameproofingagents, e.g., ammonium polyphosphates and melamine, and, if desired,corn starch, or amrnmonium polyphosphate, melamine, and expandablegraphite.

To the mixed polymer composition may also be added fillers. Of theadditives, fillers usually account for the largest weight percent of anyadditive. The amount of filler can range from 0 to 60 pbw based on 100pbw of the mixed polymer composition. Suitable fillers are conventionalorganic and inorganic fillers and reinforcing agents. Specific examplesare inorganic fillers, such as silicate minerals, for example,phyllosilicates such as antigorite, serpentine, homblendes, amphiboles,chrysotile, and talc; metal oxides, such as kaolin, aluminum oxides,titanium oxides and iron oxides; metal salts, such as chalk, barite andinorganic pigments, such as cadmium sulfide, zinc sulfide and glass;kaolin (china clay), aluminum silicate and coprecipitates of bariumsulfate and aluminum silicate, and natural and synthetic fibrousminerals, such as wollastonite, metal, and glass fibers of variouslengths. Examples of suitable organic fillers are carbon black,melamine, colophony, cyclopentadienyl resins, cellulose fibers,polyamide fibers, polyacrylonitrile fibers, polyurethane fibers, andpolyester fibers based on aromatic and/or aliphatic dicarboxylic acidesters, and in particular, carbon fibers.

The filamentous glass that may be employed as a reinforcing agent in thepresent compositions is well known to those skilled in the art and iswidely available from a number of manufacturers. Examples include "E"glass and "C" glass. The filaments are made by standard processes, e.g.,by steam or air blowing, flame blowing and mechanical pulling. Thefilament diameters generally range from about 0.003 mm to 0.15 mm.Further, the glass fibers may also be treated with functionalizedsilicon compounds to improve interaction with the polymer matrix, as iswell known to those skilled in the art. Functionalized silanes,especially alkoxy silanes may be useful in this regard.

The length of the glass filaments is also not critical. However, inpreparing molding compositions it is convenient to use the filamentousglass in the form of chopped strands of from about 0.03 to about 25 mmlong. In articles molded from the compositions on the other hand, evenshorter lengths will be encountered due to fragmentation duringcompounding.

In particular, it is preferred to used micro glass fibers having anaspect ratio of greater than 1000, more preferably from about 2000 toabout 5000, with a mean fiber diameters of 1 micron or less in order toincrease the apparent melt strength viscosity of the mixed polymercomposition at low or no shear, as well as lower its apparent melt flowviscosity at higher shear rates.

Impact modifiers may also be added. Impact modifiers generally comprisean acrylic or methacrylic grafted polymer of a conjugated diene or anacrylate elastomer, alone, or copolymerized with a vinyl aromaticcompound. On type of impact modifier is the core-shell polymer of thetype available from Rohm & Haas, for example, those sold under the tradedesignation Acryloid®. In general these impact modifiers contain unitsderived from butadiene or isoprene, alone or in combination with a vinylaromatic compound, or butyl acrylate, alone or in combination with avinyl aromatic compound. The aforementioned impact modifiers arebelieved to be disclosed in Fromuth et al., U.S. Pat. No. 4,180,494;Owens, U.S. Pat. No. 3,808,180; Farnham et al., U.S. Pat. No. 4,096,202;and Cohen et al., U.S. Pat. No. 4,260,693.

The impact modifier may comprise a two-stage polymer having either abutadiene or butyl acrylate based rubbery core and a second stagepolymerized from methylmethacrylate alone, or in combination withstyrene. Also present in the first stage are crosslinking and/orgraftlinking monomers. Examples of the crosslinking monomers include1,3-butylene diacrylate, divinyl benzene and butylene dimethacrylate.Examples of graftlinking monomers are allyl acrylate, allyl methacrylateand diallyl maleate. Additional impact modifiers are of the typedisclosed in U.S. Pat. No. 4,292,233. These impact modifiers comprise,generally, a relatively high content of a butadiene polymer grafted basehaving grafted thereon acrylonitrile and styrene. Other impact modifiersinclude, but are not limited to ethylene vinyl acetate, ethyleneethylacrylate copolymers, SEBS (styrene-ethylene-butylene styrene) andSBS (styrene-butadiene-styrene) block copolymers, EPDM (ethylenepropylene diene monomer) and EPR (ethylene propylene rubber) copolymers,etc. All of these are well known to those skilled in the art and areavailable commercially.

The mixed polymer composition of the invention can be blended with otherthermoplastic polymers. Examples of other polymers suitable for blendinginclude elastomers, polycarbonate, other types of polyesterthermoplastics, polyethylene, polypropylene, ethylene-propylenecopolymers, ethylene-vinyl acetate copolymers, polyamides, polystyrenes,styrene-butadiene copolymers, styrene-butadiene-acrylonitrilecopolymers, styrene-acrylonitrile copolymers, polyurethanes,fluoroplastics, polyphenylene oxides, polyphenylene sulfide,polybutadiene, polyolefin halides, vinyl polyhalide, butyl rubbers,silicone rubbers, and graft copolymers of polyacrylates.

In accordance with a process of the invention, a foamable compositioncomprising a mixed polymer composition is extruded under processconditions effective for the formation of a closed cell polymer, whereinthe mixed polymer composition comprises:

a) a semi-crystalline polyester composition comprising a polycondensedbranched polyester polymer, and

b) an aromatic polycarbonate polymer.

More specifically, the closed cell polyester polymer foam of theinvention can be produced by a process which comprises:

(1) feeding the mixed polymer composition along with optional additivesinto an extruder, preferably a melt extruder to conduct melt-mixing,

(2) incorporating a blowing agent into the resulting molten mixturewhile in the extruder, preferably while in the melt extruder, and

(3) extruding the mixture out of a die, preferably an annular die, toform a cellular polyester foam, preferably in the form of a sheet in acontinuous fashion.

Any extruder can be used. The process can be carried out by aplasticating extruder or a single or twin screw melt extruder. Thescrews extrude, from a metal die, the molten thermoplastic resincontaining the cells having the inert gas uniformly dispersed therein tocontinuously form a sheet in an intended shape. A single-screw extruderis used in most cases. However, in some cases, a twin-screw extruder ora multiple screw extruder having substantially the same function, isdesirable.

The mixed polymer composition and the additives may be premixed orseparately fed into the extruder hoppers. The mixed polymer compositionitself may be preblended or fed into the extruder through its individualcomponents. The order of blending or addition to the extruder is notlimited. For example, the aromatic polycarbonate may be pre-blended witha nucleating agent and fed into the extruder through a hopper, alongwith the polyester composition fed through a separate hopper.Alternatively, the aromatic polycarbonate and the polyester compositionmay be pre-blended and fed into the extruder through a single hopper.

The mixed polymer composition along with optional additives are mixedand heated in a solid transfer zone and then sent to a melting zone. Themelting zone is maintained at a temperature higher than a melting pointof the molten resin and causes melting, suction-discharging and mixingat the same time. The molten resin is carried to a melt transfer zone.In the melt transfer zone, the blowing agent is forced into the moltenresin, and sufficient mixing is conducted to uniformly disperse theblowing agent throughout the molten resin. Since the resin fed from themelting zone into the melt transfer zone which is designed to have aslightly lower temperature, it has a higher melt viscosity in the melttransfer zone. This prevents the blowing agent from flowing back throughthe extruder and escaping through the hopper. The blowing agent may befed as a gas or a liquid or generated in situ by the decomposition of achemical blowing agent.

The molten mixed polymer composition along with the blowing agent andoptional additives (extrudate) are usually extruded from thesheet-forming die using a metering pump. The metering pump and thesheet-forming die are kept at a temperature which maximizes the polymermelt strength. It is preferred to extrude a tube that can allow slitopening and thermoforming, using a circular or annular die. Otherconfigurations such as a flat sheet die can also be used. The resultingfoamed polyester sheet is cooled by air cooling, water cooling orcontacting with a chilled roll without stretching.

The mixed polymer composition of the invention, along with the blowingagent and optional additives, are suited to make a closed cell polymercomprising the mixed polymer composition comprising the polycondensedbranched polyester polymer and the aromatic polycarbonate polymer. Theclosed cell polymer of the invention can be produced as low densitysheets having a small, uniform cellular morphology while retaining highimpact strength when thermoformed into articles. The free rise densityof the closed cell polymer is not particularly limited, butadvantageously is from 0.5 g/cc or less, and is advantageously 0.5 g/ccor less as a thermoformed closed cell polymer. Preferably, the free risedensity of the closed cell polymer is 0.30 g/cc or less. The densitywill usually exceed 0.01 g/cc. The combination of a lower density andretention or improvement in impact strength, or the combination of lowerdensity and high melt strength, is unexpectedly present in the closedcell polymer of the invention compared to a comparable closed cellpolymer made with the same amount and type of ingredients but with alinear polyester or without the aromatic polycarbonate. Accordingly, thedensity of the closed cell polymer, even a thermoformed closed cellpolymer, can be reduced by 10% or more, even 15% or more, mostpreferably 20% or more, compared to a cellular polymer made with thesame ingredients except without the polycondensed branched polyester orwithout the aromatic polycarbonate.

The closed cell polymers of the invention will generally have a meancell diameter in the range of 150 μm to 250 μm. The cells aresubstantially uniformly distributed throughout the foam sheet in thisrange. The closed cell polymer has a closed cell content of at least70%, more preferably at least 80%, most preferably at least 85%.

The thickness of the closed cell polymer is not limited, but isgenerally in the shape of a sheet 5 mils to 2000 mils thick. For manyapplications, the closed cell polymer thickness will range from 30 milsto 200 mils. The closed cell polymers of the invention can be made tothicker sheets using the mixed polymer composition of the invention thancan be made using the same composition with the same amount and types ofblowing agent and other additives except without an aromaticpolycarbonate or with a linear polyester. Without being limited to atheory, it is believed that the enhanced melt strength attributable tothe aromatic polycarbonate-polycondensed branched polyester combinationprevents cell walls from rupturing, thereby promoting greater uniformityamong cell sizes, density reduction, and a larger number of cells formedto provide a thicker foam.

The impact strength of the closed cell polymer is improved over a closedcell polymer made with the same types and amounts of ingredients butwith a linear polyester or without an aromatic polycarbonate, inwhatever form, whether as a sheet or thermoformed. We have observed a20% or more improvement in room temperature Dynatup impact strength inthermoformed closed cell polymers of the invention measured at the samethickness against comparative thermoformed closed cell polymers madewith the same amount and type of ingredients except with a linearpolyester or without an aromatic polycarbonate. Since the foamed sheetsof the invention can be made substantially thicker, the improvement inDynatup impact strength can be 40% or more, even 50% or more, whenmeasured as a thermoformed article, compared to a an articlethermoformed under the same conditions but which is made with a linearpolyester composition or made without the aromatic polycarbonate. At thecolder temperature of -20° C., the dynatup impact strength of the closedcell polymer, whether as a sheet or thermoformed, is also improved,often by as much as 30% or more, compared against a comparable foam asdescribed above, at the same thickness. The improvement in impactstrength may be 40% or more.

In another embodiment, the closed cell polymers of the invention have animproved ratio of impact strength to density. In another embodiment ofthe invention, the ratio of dynatup impact strength in pounds at roomtemperature to the density of thermoformed closed cell polymers measuredin g/cc is at least 100:1, and more preferably is at least 120:1,without fillers or reinforcing agents. Measured at -20° C. in anotherembodiment, the ratio of the dynatup impact strength in pounds to thedensity of the closed cell polymer of the invention in g/cc is at least40:1, more preferably at least 50:1. These values represent asignificant improvement over comparable closed cell polymers asdescribed above.

The improvement in melt strength can be detected by the degree of dieswell in an extrusion process. We have observed a 10 fold increase ormore in die swell when extruding the composition of the inventioncompared to comparable compositions as described above. In some cases,the die swell may increase by 25 fold or more using the composition ofthe invention. In some cases, the die opening is increased to alleviatepressure increases in the extruder due to the increased die swell.

The closed cell polymer of the invention can be thermoformed into aheat-set thin article with an ordinary thermoformning device. Suchthermoforming method comprises:

1. a step of preheating the foamed polyester sheet until it is softened,and positioning it in a mold,

2. a step of pressing the preheated sheet onto a heated mold surface,

3. a step of contacting the sheet with the heated mold for a timesufficient to advance the crystallization of the polyester and heat-setthe sheet, and

4. a step of withdrawing the sheet from the cavity of the mold.

The articles made using the mixed polymer composition of the inventionare not limited. The mixed polymer compositions can be used to makenon-foamed and foamed articles, but are well adapted for extrusion intothermoformable foamed sheets which are subsequently thermoformed.Examples of suitable articles and applications, both as cut or shapedsheets or thermoformed articles, are food and oven trays, cups, andplates, each optionally and preferably foamed, on which food or liquidis heated by radiant, convection, or microwave energy; utensils, boxes,pipes, cards, yarns, fibers, films, beverage bottle preforms andmonoaxial or biaxial stretch blow moldings such as beverage bottles,photographic and packaging films, interior articles, parts for machinesand automobiles, building insulation, and flotation devices such assurfboards and boat hull foams.

The invention is now further illustrated by way of non-limitingexamples.

WORKING EXAMPLES

PET 1: is a branched polyester polymer obtained by reacting terephthalicacid with 0.2 mole percent pentaerythritol, based on the moles ofterephthalic acid, the polyester having an intrinsic viscosity of 1.2,containing 0.2 weight percent filmed silica.

PET 2: is a branched polyester polymer obtained by reacting terephthalicacid with 0.15 mole percent pentaerythritol, based on the mole percentof terephthalic acid, the polyester polymer having an intrinsicviscosity of 0.95, containing 0.2 weight percent fumed silica.

PET 3: is a branched polyester polymer obtained by reacting terephthalicacid with 0.05 mole percent pentaerythritol, based on the moles ofterephthalic acid, the polyester polymer having an intrinsic viscosityof 0.95, containing 0.2 weight percent fumed silica.

PET 4: is a branched polyester polymer obtained by reacting terephthalicacid with 0.2 mole percent pentaerythritol, based on the moles ofterephthalic acid, having intrinsic viscosity of 1.2, and containing nofumed silica.

PET 5: is a linear polyester polymer having intrinsic viscosity of 0.95commercially available from Shell Chemical Company as TFF 9506.

RDN™-1: is a nucleant package commercially available from Shell ChemicalCompany.

MAKROLON®3200: is a linear polycarbonate resin commercially availablefrom Bayer.

MAKROLON®1143: is a linear polycarbonate resin commercially availablefrom Bayer.

MAKROLON®03208: is a linear polycarbonate resin having FDA approval,commercially available from Bayer.

MAKROLON®Ku11239: is a branched polycarbonate resin with a melt flow ofless than 2.5, FDA approved and targeted for blow molding, commerciallyavailable from Bayer.

LEXAND®141: is a linear polycarbonate resin commercially available fromGeneral Electric.

In each of the examples, the polyester polymer, both branched andlinear, was processed through a twin screw extruder at different levelsof polycarbonate resin to test the melt strength of the blend over theextruder die. The closed cell polymer in the form of a microcellularfoam sheet extruded across the die was then thermoformed into bundtpans, which were subsequently tested for their Dynatup impactproperties.

In each of the examples, the extruded microcellular foam sheet was ZMDvacuum thermoformed in a metal bundt pan mold. The molding conditionsare specified in each working example.

Example 1

In this example, a Leistritz Micro 27 GL-32 twin screw extruder having32L/D twin co-rotating screws was used to pre-blend and pelletize apolycarbonate resin and a nucleant package, RDN-1 as follows: MAKROLON®polycarbonate was fed from the main feeder at a rate of 18pounds perhour, and the RDN-1 nucleant package was fed from the K-2V auxiliaryfeeder at a rate of 12 pounds per hour. The extruder temperature was setat a profile of 260-300° C. The extruder screw speed was set to 200 rpm.The resin was pelletized and dried at 90° C. for 16 hours beforeproceeding to the next step.

A closed cell polymer in the form of a foamed sheet was prepared by thefollowing procedure. The Leistritz twin screw extruder was equipped withan EDS-8 inch flat die. The dried polycondensed branched PET 1 was fedinto the extruder at the T-20 main feeder at a rate of 40 pounds perhour. The extruder screw speed was about 200 rpm and the temperatureprofile was about 260-300° C. Simultaneously, the K-2V auxiliary feederhaving a single screw delivered 1.2 pounds per hour of the pelletizedlinear polycarbonate resin-RDN-1 nucleant package at the feed port ofthe extruder to make a mixed polyester composition containing 1.8 weightpercent of the polycarbonate resin. Nitrogen gas blowing agent wasinjected into the molten mixed polyester composition at zone 8 at a gasrate of 0.1 SLM. A sample of the extruded foam sheet was collected forcharacterization. The die swell/expansion was so large, 25×, that thefoamed sheet could not be rolled up. This extremely large dieswell/expansion indicated that the branched PET/polycarbonate resinblend possessed a high melt strength. The density of the resulting foamsheet was reduced over the expected density using linear polyesters andthe same amount of blowing agent and nucleant.

The foam sheet was examined under a microscope at a magnification of100×. The cellular structure of the foam sheet was uniform, round, andclosed, with a mean diameter of about 120 microns. Furthercharacterization of the foam sheet made with the branched PET andpolycarbonate revealed the following:

                  TABLE 1                                                         ______________________________________                                        Test               Foamed PET with PC                                         ______________________________________                                        IV                 1.16                                                       COOH               5                                                          Density, g/cc      0.31                                                       Thickness, inch    0.5                                                        T.sub.m, ° C.                                                                             246.1                                                      Crystallization half time, T1/2 sec                                                              41.3                                                       Percent crystallinity                                                                            27.8                                                       ______________________________________                                    

Example 2

The same procedure used in Example 1 was used in Example 2. Thepolycarbonate/RDN-1 pellet was made by adding MAKROLON®1143 feed at 20pounds per hour from the main T-20 feeder and RDN-1 from the K-2Vauxiliary feeder at 24 pounds per hour and at a temperature profile of260-280° C. with an extruder screw speed of 200 rpm. The resultant 1:1.2PC/RDN-1 blend was pelletized and used in the next step.

Microcellular foam sheet was produced according to the same procedure asin Example 1. In this example, PET 1 was fed from the main feeder T-20at a rate of 50 pounds per hour. For the control run, the auxiliaryfeeder K-2V delivered only the nucleant package RDN-1 at a rate to yield1.2 weight percent RDN-1. The extruder temperature profile was set from269-290° C., and the screw speed was set to 250 rpm. The control sheetwas collected at 17 fpm and results of its characterization are shownbelow in Table 1.

In a second run, the branched polyester PET 1 was also fed at a rate of50 pounds per hour, except that the auxiliary feeder K-2V delivered the1:1.2 PC/RI)N-1 pellets at a rate of 1.1 pounds per hour to achieve a 1weight percent polycarbonate content and a 1.2 weight percent RDN-1content.

In each of these runs, an annular die with 1.3 inch, 70° lips were used.Nitrogen gas blowing agent was injected in each run into the moltenmixed polyester composition into the last 1/3 of the extruder at a gasrate of 0.1-0.5 SLM.

The effect of adding 1 percent polycarbonate to the branched PET was anoticeable increase in the polymer melt viscosity, which necessitated a5 percent increase in a die opening at the extruder head to alleviatethe pressure increase in the extruder. After the foam sheet wascharacterized, it's Dynatup impact strength was tested by thermoformingthe sheet on a ZMD vacuum molding machine under the followingconditions: sheet temperature was raised to 157° C., the moldtemperature of 199° C. for 11 seconds, and the oven temperature was setat 102° C. for 5.6 seconds. The results are set forth in Table 2.

                  TABLE 2                                                         ______________________________________                                                           Density   Thick-                                                              (g/cc),   ness,  Impact, lb. at                            Material   IV      Free Rise mils   Ambient Temp.                             ______________________________________                                        PET 1 + 0 wt. %                                                                          0.924   0.428     10     3.3                                       PC (Control)                                                                  PET 1 + 1 wt. %                                                                          1.062   0.412     10     7.6                                       PC                                                                            ______________________________________                                    

The results indicate that the impact strength of the thermoformed partmade by adding as little as 1 weight percent of polycarbonate resin to abranched polyethylene terephthalate resin more than doubled over thesame polyester resin made in the absence of the aromatic polycarbonate.The melt viscosity of the resin was also dramatically improved uponaddition of 1 weight percent polycarbonate resin, so much so that thedie opening had to be increased to alleviate the pressure increase inthe extruder.

Example 3

In this example, the processing conditions of Example 2 were used,including the manufacture of the polycarbonate/RDN-1 blend at a 1:1.2ratio, respectively, the preparation of the control, and the use ofMAKROLON®1143 linear polycarbonate resin. The difference in thisexample, however, was that PET 2 was used as the polyester resin. Thispolyester resin contained a lower amount of reacted polycondensationbranching agent than PET 1, as well as a lower intrinsic viscosity.

In this example, the control, prepared without any polycarbonate resin,was very difficult to manufacture as a continuous foam sheet across thecooling can due to its marginal melt strength. The extrudate tended totear across the cooling can at different intervals, rather than floweasily across the annular die to make a continuous foam sheet. Theaddition of 1 weight percent polycarbonate resin, however, had the samedramatic effect as in Example 2, and it became much easier to form andtake up a continuous foam sheet. Samples from both sheets werecharacterized, and subsequently vacuum thermoformed as an Example 2 totest their impact strength. The results are set forth in Table 3.

                  TABLE 3                                                         ______________________________________                                                        Thickness,                                                                             Density, Impact lb.                                  Material        mils     g/cc, fr.                                                                              at Ambient                                  ______________________________________                                        PET 2 + 0 wt. % PC (Control)                                                                  17       0.369    6.8                                         PET 2 + 1 wt. % PC                                                                            16       0.372    8.6                                         ______________________________________                                    

The results again indicate an improvement in dynatup impact strength byemploying an aromatic polycarbonate in an amount as little as 1 weightpercent, compared to the same thermoformed polyester having nopolycarbonate resin.

Example 4

In this example, pellets of polycarbonate/RDN-1 were prepared on theLeistritz extruder at a PC/RDN-1 ratio of 10:1.2. The polycarbonate feedwas delivered at a rate of 30 pounds per hour on the main T-20 feederand was combined in the extruder with the RDN-1 at a rate of 3.6 poundsper hour fed from the auxiliary K-2V feeder. The extruder temperatureprofile was set to 260-280° C. The material was pelletized and driedunder vacuum at 90° C. for 16 hours before using it in the next stepmaking a foam sheet.

The microcellular foam sheet was prepared as follows: dried PET 3,containing only 0.05 percent mole percent pentaerytlritol and possessingan intrinsic viscosity of 0.95, was fed from the main feeder T-20 at arate of 40 pounds per hour.

In the preparation of a control without the polycarbonate, where only1.2 weight percent of RDN-1 was simultaneously fed from the auxiliaryfeed, it was impossible to make a continuous foam sheet using theextruder conditions described in Example 2. It was plainly evident thatthe polycondensed branched polyester resin alone did not possesssufficient melt strength to be pulled across an annular die withoutcontinually ripping and tearing. In a second run, the dried PET 3, alsofed from the main feeder at 40 pounds per hour, was combined in theextruder with the PC/RDN-1 pellet blends delivered at a rate of 4.4pounds per hour, to obtain a final polycarbonate loading of 10 percent.Unlike the control, the extrudate could be pulled across the annular dieto form a continuous microcellular foam sheet. The sheet ischaracterized, and subsequently thermoformed as an Example 2 into atray. The Dynatup impact of the thermoformed part was also tested. Theresults are reported in Table 3.

                  TABLE 4                                                         ______________________________________                                        Material        Density, g/cc                                                                           Impact, lb. at Ambient                              ______________________________________                                        PET 3 + 0 wt. % PC (Control)                                                                  --        --                                                  PET 3 + 10 wt.% PC                                                                            0.50      5.6                                                 ______________________________________                                    

This example shows that it is possible to use polycarbonate to enhancethe properties of a marginally polycondensed branched PET having a lowintrinsic viscosity, as well as prepare molded trays with acceptableproperties. It is significant that in the absence of polycarbonate, nocontinuous foam sheet could be produced using a polyester having thislow level of polycondensation branching agent. However, the addition ofan aromatic polycarbonate significantly enhances the melt strength, evenof polyester resins containing low levels of polycondensation branchingagent.

Example 5

This example is illustrated to demonstrate that the combination of anaromatic polycarbonate with a linear PET cannot be used to succesfullyextrude a continuous foam sheet. In this control example, the processingconditions of Example 2 were used. PET 5, a linear polyester, was usedalong with 1.0% of an aromatic polycarbonate and 1.2% of the RDNnucleant package. Nitrogen gas in the amount described in Example 2 wasused.

The polyester composition could not be extruded across the annular diewithout ripping. Of the small segments of sheet that were made, theappearance of the sheet was open-cell, lace like, with a cheese clothappearance having no mechanical properties and useless for anythermoformed application.

Example 6

In each of the runs in this example, a Leistritz Micro 27 extruder with40L/D co-rotating screws was used to prepare a microcellular foam sheet.The polycarbonate resin and the RDN-1 nucleant package were notpreblended and pelletized, but rather fed as individual streams, thepolycarbonate resin through the K-2V auxiliary feeder and the RDN-1nucleant package through the Conair auxiliary feed. The main feeder T-20equipped with one screw delivered the polyester resin. A melt pump andan annular die with 1.3 inch, 70° lips were used.

The main feeder T-20 delivered PET 4 at a rate of 30 pounds per hour.The extruder pressure varied with the amount of PC added. The amount ofnitrogen gas delivered ranged from 0.1-0.4 SLM. The extruder temperatureprofile was set from 215-300° C. The extrudate temperature was about258° C. The sheet take up was set at a rate of about 5-7 fpm. The typeof polycarbonate used is reported in Table 5. The rate of polycarbonatedelivered was 0.45-0.9 pounds per hour. The rate of RDN-1 added to thepackage delivered was 0.36 pounds per hour.

A control made of PET 4 containing 1.2 weight percent of the RDN-1nucleant package was prepared according to the conditions above. Becauseof the high degree of crosslinking present in PET 4, it was possible tomake a foam sheet, however, the processing window was narrow and it wasextremely difficult to yield a continuous thick foam sheet. Afterseveral attempts, the best and highest gauge sheet prepared from thecontrol was selected for characterization and thermoforming into bundtpans of 55 mil thicknesses. The result is reported in Table 5 below.

PET 4 with 1.2 weight percent RDN-1 and the amount and type ofpolycarbonate resin specified in Table 5 was prepared according to theconditions specified above. In these runs, a high die swell was observedeven at low amounts (less than 0.5 mole percent) of aromaticpolycarbonate. All the modifications using the PC resins gave comparableto or, in most cases, thicker foam sheets. These foam sheets were alsoZND vacuum thermoformed according to the conditions specified above. Theresults are reported in Table 5 below.

                                      TABLE 5                                     __________________________________________________________________________                     DYNATUP IMPACT                      DENSITY                                   ROOM TEMP          -20° C.   (g/cc),                                                                            THICK-                                      Ratio                                                                             Energy                                                                            Ratio                                                                             Max  Ratio                                                                             Energy                                                                            Ratio                                                                             Free NESS                SAMPLE   PC      Max. Load lb.                                                                        L/D In-lb                                                                             E/D Load lb.                                                                           L/D In-lb                                                                             E/D Rise (mils)              __________________________________________________________________________    CONTROL  Macrolon ® 3208                                                                   27.3   79.4                                                                              3.3 9.59                                                                              8.6  25  0.7 2.0 0.344                                                                              55                  PET4 + 0.0% PC                                                                PET4 + 0.32% PC                                                                        Macrolon ® 3208                                                                   31.9   116 4.1 15  7.8  28.4                                                                              0.6 2.2 0.274                                                                              55                  PET4 + 0.53% PC                                                                        Macrolon ® 3208                                                                   33.6   144 4.5 19.3                                                                              17.1 73  1.7 7.3 0.233                                                                              68                  PET4 + 0.75% PC                                                                        Macrolon ® 3208                                                                   43.8   178 5.9 24  16.4 66  1.3 5.3 0.246                                                                              70                  PET4 + 1.5% PC                                                                         Macrolon ® 3208                                                                   36.6   108 5.5 16.3                                                                              14.4 42.6                                                                              1.2 3.5 0.338                                                                              42                  PET4 + 0.32% PC                                                                        Macrolon ®                                                                        37.8   126 4.7 15.8                                                                              11.7 34.3                                                                              0.9 3.0 0.298                                                                              52                  (Branched PC)                                                                          Ku11239                                                              PET4 + 0.53% PC                                                                        Macrolon ®                                                                        32.6   93.1                                                                              4.2 12  14.4 41  1.2 3.4 0.351                                                                              64                           Ku11239                                                              __________________________________________________________________________

PET has an inherently low melt strength, which contributes to poorfoaming properties. Another shortcoming of PET is that it has poorimpact strength. By adding a small amount of polycarbonate resin to apolycondensed branched PET, all of these shortcomings have beenovercome.

As noted in Example 5, linear PET cannot be extruded into a continuousfoamed sheet.

Even the addition of 1.0 wt. % of a polycarbonate to a linear PET, asnoted in Example 5, was insufficient to enable the extrudate to processwithout ripping, and could not produce a closed cell polymer or a foamof any use. As observed in Example 4, using solely a lightlypolycondensed branched (0.05 mole%) and a low IV (0.95) PET wasinsufficient to create a melt which extrudes well and consistently froman annular die, over a cooling can, and into a foamed sheet, due to itsinsufficient melt strength. As seen from the control in Example 6,increasing the amount of crosslinking, while sufficient to allow one toextrude continuous foam sheet with difficulty, could not produce a foamsheet of low density and a thermoformed article of high impact strength.

It was only upon combining a polycondensed branched polyester with anaromatic polycarbonate that a die swell, a continuous thick low densityfoam sheet, and a thermoformed article having high impact strength wereexperienced.

The results in Table 5 indicate that the addition of polycarbonate to apolyester composition containing polycondensed branched PET reduced thedensity of the foamed product by as much as about 30%, compared to theControl in which only the polycondensed branched PET was used. Due tothe density reduction and increased melt strength, much thicker foamedsheets were made compared to the control. Accordingly, by addingpolycarbonate to a polycondensed branched PET, less PET resin is neededto make a foamed sheet having the equivalent thickness as a foamed sheetmade from a branched PET. Further, at an equivalent thickness, thedensity of the foamed sheet made with polycarbonate and a polycondensedbranched PET is substantially reduced over a foamed sheet made simplyfrom polycondensed branched PET.

The impact strength of the thermoformed sheets made with a polycondensedbranched PET in combination with a polycarbonate, tested at both roomtemperature and at -20° C., was substantially improved over thethermoformed sheets made with polycondensed branched PET in the absenceof polycarbonate. As indicated in Table 5, the impact strength ofthermoformed sheets at room temperature and at -20° C. made withpolycondensed branched PET and both linear and branched polycarbonateare substantially improved over the Control polycondensed branched PET.Unexpectedly, the polycondensed branched PET/polycarbonate thermoformedsheets had a better impact strength over the Control even though the PETpolycarbonate thermoformed sheets had a lower density. And thisimprovement in impact strength was evident not only at lower densities,but also at equivalent sheet thicknesses of about 55 mils or thinner.Accordingly, foamed sheets can now be made at equivalent thicknesseswith a simultaneous density reduction and improvement in impactstrength.

The results in Table 5 also indicate a steady increase in impactstrength through 0.75% polycarbonate followed by a tapering off andsomewhat of a decline at a point between 0.75% and 1.5% polycarbonate,for polyester compositions having an intrinsic viscosity at 1.2.Nevertheless, the impact strength at 1.5% polycarbonate wassignificantly better than the physical properties of the Control. Thedensity profile followed this same pattern.

The density of these runs was normalized by taking the ratio of impactstrength at max load or its energy to the density. The results show asignificant improvement in the impact strength of the polycondensedbranched PET/polycarbonate combination relative to a polycondensedbranched PET without polycarbonate.

The results in Table 5 also indicate that the branched polycarbonate ata low loading of only 0.3% produced a thermoformed sheet having betterimpact strength than one made with a linear polycarbonate, both at roomtemperature and at -20° C. This suggests that at low loadings, abranched polycarbonate/polycondensed branched PET combination has thepotential to provide better properties than the improvement realizedwith a linear polycarbonate/polycondensed branched PET combination.

What we claim is:
 1. A foamable composition comprising a blowing agentand a mixed polymer composition comprising:a) a semi-crystallinepolyester composition comprising a polycondensed branched polyesterpolymer, and b) an aromatic polycarbonate polymer.
 2. The foamablecomposition of claim 1, wherein said polymer composition is athermoplastic composition.
 3. The foamable composition of claim 1,wherein the polycondensed branched polyester polymer comprises areaction product of 0.01 to 5.0 mole % of a polycondensation branchingagent based on the total moles of polyester polymer monomers.
 4. Thefoamable composition of claim 3, wherein the polycondensed branchedpolyester polymer comprises the reaction product of 0.01 to 1.0 molepercent of a polycondensation branching agent based on the total molesof polyester polymer monomers.
 5. The foamable composition of claim 4,wherein the polycondensation branching agent has an average nominalfunctionality of greater than
 2. 6. The foamable composition of claim 5,wherein the polycondensation branching agent comprisestrimethylolpropane, trimethylolethane, pentaerythritol, glycerine,sucrose, or mixtures thereof.
 7. The foamable composition of claim 5,wherein said polycondensation branching agent comprises trimesic acid,trimellitic acid, pyromellitic acid, citric acid, benzophenonetetracarboxylic acid, the anydrides or salts thereof, or mixturesthereof.
 8. The foamable composition of claim 5, wherein saidpolycondensed branched polyester polymer comprises an aromaticpolycondensed branched polyester polymer.
 9. The foamable composition ofclaim 1, wherein the polycondensed branched polyester polymer possessesan IV of 0.65 to 1.75 g/cc.
 10. The foamable composition of claim 9,wherein the polycondensed branched polyester polymer possesses and IV of0.95 to 1.5 g/cc.
 11. The foamable composition of claim 1, wherein themixed polymer composition comprises polycarbonate in an amount of from0.05 wt. % to 10 wt. % based on the weight of the mixed polymercomposition.
 12. The foamable composition of claim 11, wherein the mixedpolymer composition comprises polycarbonate in an amount of from 0.1 wt.% to 2.0 wt. %, based on the weight of the mixed polymer composition.13. The foamable composition of claim 12, wherein the mixed polymercomposition comprises polycarbonate in an amount of from 0.2 to 1.0 wt.% based on the weight of the mixed polymer composition.
 14. The foamablecomposition of claim 1, wherein the aromatic polycarbonate comprises alinear aromatic polycarbonate.
 15. The foamable composition of claim 14,wherein the aromatic polycarbonate is derived from Bisphenol A.
 16. Thefoamable composition of claim 1, wherein the aromatic polycarbonate isrepresented by the structure: ##STR2## wherein A is a multivalentaromatic radical derived from the polyhydric phenol used in thepreparation of the polymer, and n represents the number of repeatcarbonate units.
 17. The foamable composition of claim 16, wherein A isderived from a polynuclear aromatic compound comprising two or morehydroxy radicals, each directly bonded to a carbon atom of the aromaticnucleus.
 18. The foamable composition of claim 1, wherein the aromaticpolycarbonate comprises a branched aromatic polycarbonate.
 19. Thefoamable composition of claim 18, wherein the amount of branchedaromatic polycarbonate is at least 0.2 wt. % based on the weight of thepolymer composition.
 20. The foamable composition of claim 18, whereinthe branching on said branched aromatic polycarbonate is a C₂ -C₁₀alkyl, C₅ -C₁₅ cycloalkyl, or a C₆ -C₂₀ aryl, aralkyl, or alkarylaromatic branch.
 21. The foamable composition of claim 1, wherein saidpolycondensed branched polyester polymer is semi-crystalline at 25° C.and 1 atmosphere.
 22. The foamable composition of claim 1, wherein themixed polymer composition comprises a semi-crystalline polyestercomposition in an amount of 90 wt. % or more, based on the weight of themixed polymer composition.
 23. The foamable composition of claim 22,wherein the mixed polymer composition comprises the aromaticpolycarbonate in an amount of from 0.2 wt. % to 1 wt. %, based on theweight of the mixed polymer composition.
 24. The foamable composition ofclaim 1, said mixed polymer composition further comprising a nucleatingagent, said foamable composition optionally comprising a filler,colorant, reinforcing agent, and/or an antioxidant.
 25. The foamablecomposition of claim 1, wherein the amount of said mixed polymercomposition comprises at least 40 wt. % based on the weight of thefoamable composition.
 26. The foamable composition of claim 25, saidmixed polymer composition further comprising a nucleating agentcomprising a polyolefin, a polyethylene polymer, a polyhaloethylenepolymer, or mixtures thereof.
 27. A closed cell polymer comprising amixed polymer composition comprising:a) a semi-crystalline polyestercomposition comprising a polycondensed branched polyester polymer, andb) an aromatic polycarbonate polymer.
 28. The closed cell polymer ofclaim 27, said closed cell polymer has a free rise density of 0.5 g/ccor less.
 29. The closed cell polymer of claim 28, wherein said free risedensity ranges from 0.01 g/cc to 0.30 g/cc.
 30. The closed cell polymerof claim 27, wherein the free rise density of the closed cell polymer isreduced by 10% or more compared to a cellular foam made with the sameingredients under the same process condition, except with only linearpolyester polymers as the polyester polymers or without an aromaticpolycarbonate.
 31. The process of claim 30, wherein the reduction infree rise density is 15% or more.
 32. The closed cell polymer of claim27, having a mean cell diameter within the range of from 150 μm to 250μm.
 33. The closed cell polymer of claim 27, having a closed cellcontent of at least 70%.
 34. The closed cell polymer of claim 33,wherein the closed cell content is at least 80%.
 35. The closed cellpolymer of claim 27, wherein said closed cell polymer has a Dynatupimpact strength which is at least 20% greater at room temperaturecompared to the same closed cell polymer at the same thickness madeunder the same extruder process conditions using the same ingredientsexcept with only linear polyester polymers as the polyester polymers orwithout an aromatic polycarbonate.
 36. The closed cell polymer of claim27, wherein said closed cell polymer has a Dynatup impact strength whichis at least 30% greater at -20° C. compared to the same closed cellpolymer at the same thickness made under the same extruder processconditions using the same ingredients except with only linear polyesterpolymers as the polyester polymers or without an aromatic polycarbonate.37. The closed cell polymer of claim 27, wherein said closed cellpolymer has a Dynatup impact strength which is at least 40% greater atroom temperature compared to a closed cell polymer made under the sameextruder process conditions using the same ingredients except with onlylinear polyester polymers as the polyester polymers or without anaromatic polycarbonate.
 38. The closed cell polymer of claim 27, whereinsaid closed cell polymer has a Dynatup impact strength which is at least40% greater at -20° C. compared to a closed cell polymer made under thesame extruder process conditions using the same ingredients except withonly linear polyester polymers as the polyester polymers or without anaromatic polycarbonate.
 39. The closed cell polymer of claim 27, whereinsaid closed cell polyester foam has a free rise density, and isthermoformed into a thermoformed polymer having a Dynatup impactstrength, and the ratio of the Dynatup impact strength in pounds at roomtemperature to the density of thermoformed closed cell polymers measuredin g/cc is at least 100:1, as measured without fillers or reinforcingagents.
 40. The closed cell polymer of claim 39, wherein said ratio isat least 120:1.
 41. The closed cell polymer of claim 40, wherein saidclosed cell polyester foam has a free rise density, and is thermoformedinto a thermoformed polymer having a Dynatup impact strength, and theratio of the Dynatup impact strength in pounds at -20° C. to the densityof thermoformed closed cell polymers measured said ratio is at least40:1 in the absence of fillers and reinforcing agents.
 42. The closedcell polymer of claim 41, wherein said ratio is at least 50:1.
 43. Theclosed cell polymer of claim 27, comprising a thermoformable sheet. 44.The closed cell polymer of claim 27, comprising a plate, an oven tray, acup, a bottle, a utensil, a card, a box, or a pipe.
 45. The closed cellpolymer of claim 27, comprising building insulation.
 46. The closed cellpolymer of claim 27, comprising an oven tray.
 47. The closed cellpolymer of claim 27, comprising a flotation device.
 48. The closed cellpolymer of claim 47, comprising a surfboard foam or a boat hull foam.49. The closed cell polymer of claim 27, wherein said cells are madeusing nitrogen as a blowing agent.